Gas separation by pressure swing adsorption (PSA) is achieved by coordinated pressure cycling and flow reversals over an adsorbent bed which preferentially adsorbs a more readily adsorbed component relative to a less readily adsorbed component of the mixture. The total pressure is elevated during intervals of flow in a first direction through the adsorbent bed from a first end to a second end of the bed, and is reduced during intervals of flow in the reverse direction. As the cycle is repeated, the less readily adsorbed component is concentrated in the first direction, while the more readily adsorbed component is concentrated in the reverse direction.
Many prior art PSA systems have low energy efficiency, because feed gas for adsorber pressurization as well as for the high pressure production step is provided by a compressor whose delivery pressure is the highest pressure of the cycle. Energy expended in compressing the feed gas used for pressurization is then dissipated in throttling across valves over the instantaneous pressure difference between the adsorber and the high pressure supply. Similarly, in vacuum swing adsorption (VSA) where the lower pressure of the PSA cycle is established by a vacuum pump exhausting gas at that pressure, energy is dissipated in throttling over valves during countercurrent blowdown of adsorbers whose pressure is being reduced. A further energy dissipation occurs in throttling of light reflux gas used for purge, equalization, cocurrent blowdown and product pressurization or backfill steps. The energy dissipation in irreversible throttling becomes more important when such throttling takes place over larger pressure differences between an adsorber and a feed source or an exhaust sink.
Energy efficiency has been improved in more modern VSA air separation systems, by using feed compressors (or blowers) whose delivery pressure follows the instantaneous pressure of an adsorber being pressurized, and by using vacuum pumps whose suction pressure follows the instantaneous pressure of an adsorber undergoing countercurrent blowdown. In effect, the feed compressor rides each adsorber in turn to pressurize it with reduced throttling losses, and likewise the vacuum pump rides each adsorber in turn to achieve countercurrent blowdown with reduced throttling losses. In such systems, each feed compressor can only supply gas to a single adsorber at any time, and each vacuum pump can only exhaust a single adsorber at a time. The working pressure in each such feed compressor or vacuum pump will undergo large variations, stressing the machinery and causing large fluctuations in overall power demand. Further, compression efficiency is compromised by the unsteady operating conditions.
Since centrifugal or axial turbomachinery cannot operate under such unsteady conditions, rotary positive displacement machines are typically used in VSA systems. However, such machines have lower efficiency than modern centrifugal turbomachinery working under steady conditions, particularly for larger plant ratings (e.g. 50 tons per day oxygen VSA systems). Further, scale up above single train plant capacities of about 80 tons per day oxygen is inhibited by the maximum capacity ratings of single rotary machines.
Other modern VSA air separation systems have used multiple individual impellers to increase the enthalpy of the individual streams. However, these latter systems increase system complexity and capital cost. Furthermore, machine efficiency is reduced since the flow rates are smaller for each machine.
Accordingly, there is a need for centrifugal turbomachinery which can be used in PSA and VSA gas separation processes for maintaining steady conditions of gas flow and pressure, while minimising energy dissipation in irreversible throttling.